Rheological properties of highly flowable mortar containing limestone filler-effect of powder content and W/C ratio

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Abstract

The effect of a limestone filler addition in superplasticized cement mortar was investigated. The mixtures considered in this study are highly fluid, yet stable mortars that can be used to proportion self-consolidating concrete (SCC). All the mixtures were proportioned with a fixed unit water content of 250 l and various water–cement ratios varying from 0.35 to 0.45. A limestone filler with a specific surface area of 480 m2/kg was used at different addition percentages.

This paper reports test results leading to the recommendation of suitable powder contents that can be used to proportion mortar mixtures containing a limestone filler and achieving adequate rheological properties. Test results show that the effect of limestone filler is mainly affected by the W/C and the limestone filler content in use. For a given W/C, the addition of a limestone filler within a certain range did not affect fluidity. However, beyond a critical dosage, the incorporation of some limestone filler resulted in a substantial increase of the viscosity of mortar. An accurate model that can be used to predict the viscosity of such mixtures is proposed and validated on various mixtures.

Introduction

Self-consolidating concrete (SCC) is a new generation of high-performance concrete that can exhibit high deformability and can flow into place under its own weight without any external consolidation and with limited signs of segregation. Deformability of SCC refers to the ability of a fresh mixture to deform and undergo change in shape and to flow around obstacles while maintaining good suspension of coarse particles in the matrix, thus avoiding arching near obstacles and blockage during flow. The design concept of SCC is mainly based on the combination of high deformability and segregation resistance to achieve self-consolidation, facilitate casting, and improve in-situ performance.

The successful development of SCC requires a careful control of mixture parameters, such as the rheological properties of matrix, content of supplementary cementitious materials (SCM), as well as the content and particle size distribution of coarse aggregates. SCC is a multiphase material in which coarse aggregates (solute) are suspended in highly flowable mortar (solution). The flow properties and segregation resistance of SCC are consequently controlled via proper adjustment of the rheology of the mortar and adequate selection of the content of coarse aggregates. For a given type and content of coarse aggregates, proper viscosity of the matrix is required to ensure the homogenous suspension of particles and reduce interparticle collision that can cause a local increase of internal stress and flow resistance [1], [2], [3], [4], [5]. The viscosity of cement-based material can be improved by decreasing the water–SCM ratio (w/cm) or using a viscosity-enhancing agent. It can also be improved by increasing the cohesiveness of the paste through the addition of filler, such as limestone [1], [2], [3], [5], [6]. However, excessive addition of fine particles can result in a considerable increase in the specific surface area of the powder, which results in an increase of water demand to achieve a given consistency. On the other hand, for a fixed water content, high powder volume (Pv) increases interparticle friction due to solid–solid contact. This may affect the ability of the mixture to deform under its own weight and pass through obstacles [1], [2], [5], [7].

The Japan Society of Civil Engineers Specifications recommend to use an SCM content ranging between 16% and 19%, by volume of concrete, to proportion SCC [1]. SCC mixtures made with a ternary blended cement containing 19% of SCM, by volume, were successfully proportioned for casting highly congested structural sections [5]. It should be noted, however, that the required volume of powder is controlled by the particle size distribution of the aggregate, and for a given particle size distribution of aggregate, the volume of powder must be sufficient to fill the interstitial voids, thus reducing interparticle friction between coarse aggregate.

An advantage of incorporating SCM in SCC lies in the resulting enhancement of particle distribution, reduction of the risk of thermal cracking, as well as the improvement of certain mechanical and rheological properties [8], [9]. For example, the use of limestone filler can enhance many aspects of cement-based systems through physical or chemical effects. Some physical effects are associated with the small size of limestone particles, which can enhance the packing density of powder and reduce the interstitial void, thus decreasing entrapped water in the system. For example, the use of a continuously graded skeleton of powder is reported to reduce the required powder volume to ensure adequate deformability for concrete [10]. Chemical factors include the effect of limestone filler in supplying ions into the phase solution, thus modifying the kinetics of hydration and the morphology of hydration products [8].

Partial replacement of cement by an equal volume of limestone filler with a specific surface area ranging between 500 and 1000 m2/kg resulted in an enhancement in fluidity and a reduction of the yield stress of highly flowable mortar [11], [12]. Other investigations have shown that partial replacement of cement by an equal volume of limestone filler varying from 5% to 20% resulted in an enhancement of the fluidity of high-performance concrete having a W/C ratio ranging between 0.35 and 0.41 [13]. This improvement may be due to the increase in W/C or in paste volume. Indeed, for a given water content, partial replacement of cement by an equal volume of a filler results in an increase in W/C. On the other hand, partial replacement of cement by an equal mass of limestone filler results in an increase of powder content, i.e., an increase in paste volume. For example, the partial substitution of cement by 40%, by mass, of limestone filler having a specific gravity of 2.7 yields to a 17% increase in powder volume. Therefore, the sole physical effect of limestone filler on the rheological properties of equivalent SCC mortar is still not well established.

The main objective of the present study is to investigate the effect of a limestone filler addition on the rheological properties of “equivalent” SCC mortar made with a fixed water content and different W/C ratios of 0.35, 0.40, and 0.45. Results of this study can therefore provide a guideline to determine the suitable powder volume necessary to impart adequate fresh properties of a superplasticized mortar that can be used to design SCC. Furthermore, an analytical model that can predict the viscosity of mortar is developed and validated.

Section snippets

Research significance

SSC contains a relatively higher powder content than conventional concrete does. Fillers, such as limestone, are used as a portion of total SCM to overcome the increase in temperature due to the hydration of cement and enhance certain properties of SCC. Test results presented in this paper highlight the effect of limestone filler addition on the properties of fresh equivalent SCC mortar proportioned with a fixed water unit content and various W/C values. Suitable powder volumes to achieve

Materials

All mixtures investigated in this study were systematically proportioned using a Japanese ordinary portland cement with a Blaine fineness of 350 m2/kg. A limestone filler with a specific gravity of 2.80 and a Blaine fineness of 480 m2/kg was used. The chemical and physical properties of these materials are given in Table 1.

A well-graded sand with a fineness modulus of 2.6, a specific gravity of 2.65, and an absorption value of 1.2% was employed. The sand was a combination of sea and crushed

Effect of HRWR on relative flow area and relative V-funnel FT

The mixture proportioning and test results obtained in Phase 1 are summarized in Table 2. Fig. 1, Fig. 2, Fig. 3 present the variation of Γ and Φ parameters for mixtures proportioned with a W/C of 0.35, 0.40, and 0.45, respectively. For each W/C, limestone filler is added at minimum and maximum dosages to ensure a powder volume between 22% and 30%, by volume of mortar (these percentages correspond approximately to 16% and 19%, respectively, by volume of a concrete containing 27% to 33% of

Conclusions

Based on the results presented in this paper, the following conclusions can be warranted:

  • 1.

    The physical effect of limestone filler depends on mixture parameters, especially the W/C and the addition dosage of limestone filler.

  • 2.

    For a given W/C and HRWR dosage, the addition of limestone filler within a given range did not affect the fluidity of the mixture. However, when used beyond a critical dosage, the addition of limestone filler resulted in a substantial increase in viscosity.

  • 3.

    Suitable powder

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